Dark trace cathode ray tube with photochromic image screen



Z. J. KISS Dec. 15, 1970 DARK TRACE CATHODE RAY TUBE WITH PHOTOCHROMICIMAGE SCREEN Filed Jan. 24, 1968 SWS@ x. .,m

.w Q 25m. h hmkowwwwwwwwnw conm. o www l' N .n NN QNSNE m Tx um QEN UQl1 W mxmu m M l) -mw A 7 r E E H N :l N. Z Nu U. Q QQ .muwm E w mnmUnited States Patent Olhce DARK TRACE CATHODE RAY TUBE WITH PHOTOCHROMICIMAGE SCREEN Zoltan I. Kiss, Belle Mead, NJ., assignor to RCACorporation, a corporation of Delaware Filed Jan. 24, 1968, Ser. No.700,148 Int. Cl. F21k 2/ 00; H01j 29/14, 29/26 U.S. Cl. 313-91 11 ClaimsABSTRACT OF THE DISCLOSURE In a cathode ray tube, an image screen iscomprised .of a non-luminescent, photochromic material characterized inthat a stable, visible, dark trace image formed on this maten'al iscompletely erasable by a photon induced electron charge transfertransition.

BACKGROUND OF THE INVENTION Dark trace cathode ray tubes are known inthe art. U.S. Pat. No. 2,432,908 issued to Humboldt W. Leverenzdescribes a dark trace cathode ray tube comprising an alkali halidetarget material. `In this tube, electron beam bombardment of the alkalihalide material creates color centers in the material. These colorcenters form an image which can be viewed by transmitted or reflectedlight. In order to completely erase this image and dissipate the colorcenters, one either must wait for the normal thermal decay or provideheat to the alkali halide to accelerate erasure. Heat for erasure hasbeen provided by various means including heating filaments, ultra violetand infra red light and high intensity illumination within theabsorption band of the alkali halide target. However, erasure due toheat presents two problems. One problem is that the erasure time isoften longer than desirable and the second problem is that a new imagecannot be formed on the target until the target has lost a substantialproportion of the heat applied to it during erasure. Hence, it isdesirable to have a cathode ray tube in which an image can be formed byan electron beam and can be erased by means other than thermal decay.For example, images that can be erased due to quantized charge transfertransitions would be superior to prior art dark trace cathode ray tubesin which the images are erased by heating.

In U.S. Pat. No. 2,563,472, issued to Humboldt W. Leverenz, a cathoderay tube having a scotophor target is disclosed. The cathode ray tubedisclosed therein has inducible and eradicable absorption bands in theinvisible regions of the spectrum. Images formed on this type of tubeare invisible to the eye and must be used in conjunction with an imageconverter for the images to be seen by an observer. Some invisible tracecathode ray tubes as well as some visible dark trace cathode ray tubeshave the disadvantage of luminescing when exposed to cathode rays. Thisluminescence is unwanted and distracting when used in the screen of adark trace cathode ray tube.

SUMMARY OF THE INVENTION In a cathode ray tube, an image screen iscomprised of a non-luminescent, photochromic material characterized inthat a stable, visible dark trace image formed on said material by anelectron beam is completely erasable by a photo-induced electron chargetransfer transition.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1-3 are graphicalrepresentations of the absorption characteristics of severa1 novelscreen materials before and after erasure.

3,548,236 Patented Dec. 15, 1970 A photochromic material as used hereinis a material having photon inducible and photon eradicable absorptionbands in the visible regions of the electromagnetic spectrum. In theparticular inorganic crystalline photochromic materials disclosedherein, the absorption bands are also inducible by electron beambombardment of the photochromic material. The mechanism of erasure ofthe absorption bands in these materials involves a photon-inducedelectron charge transfer transition. In this mechanism, absorption of aphoton induces electron transfer from one trap site in the photochromiccrystal to another site in the photochromic crystal. This electrontransfer causes erasure of a previously induced absorption band.Generally, the photochromic materials useful in the novel dark tracecathode ray tubes disclosed herein are non-cathodoluminescent.

Examples of such photochromic materials are: a1- kaline earth titanatescontaining small quantities of transition metal ions, such as, strontiumtitanate doped with iron and/or molybdenum, and calcium titanate dopedwith iron and/or molybdenum; sodalite preferably containing smallquantities of transition metal ions, such as sodalite doped with iron;alkaline earth uorides containing small amounts of divalent rare earthions, such as, calcium fluoride doped with cerium, lanthanum, gadoliumor terbium; and molybdenum trioxide.

FIG. l is a graphical representation of characteristic absorptionproperties of one millimeter thick calcium titanate crystal doped with0.05% iron and 0.1% molybdenum. Curve 1 shows the absorptioncharacteristics of this material before being colored by an electronbeam. This curve is identical to the absorption characteristics obtainedafter erasure or bleaching of a previously colored crystal. Such erasureis accomplished by exposing the colored crystal to high intensity lightin the absorption band. Preferably light of about 4300 A. is used. Curve2 shows the absorption characteristics of the calcium titanate afterbeing colored by an electron beam. The crystal colored by an electronbeam appears almostblack to the eye while the uncolored or erasedmaterial as shown in Curve 1 appears transparent and relatively neutralin color to the eye.

FIG. 2 is a graphical representation of the characteristic absorptionproperties of a sodalite or hackmenite crystal doped with iron. Thiscrystal in its uncolored or bleached state does not absorb light in thevisible region of the spectrum, as indicated by Curve 3, and appearsneutral and completely transparent to the eye. After electron beambombardment, the crystal takes on a magenta color in the regionsbombarded by the electron beam and these regions have an absorptioncharacteristic as shown in Curve 4. Erasure of this magenta color isaccomplished by exposing the photochromic to light anywhere in theinduced absorption band. Preferably, light of about 5200 A. is usedsince absorption and erasure efficiency is greatest at about thiswavelength. In all of the novel materials, the efficiency of thephoton-induced electron transfer transition which causes erasure issubstantially less than the efficiency of writing the image onto thecrystal. Due to this fact, normal room light will not cause substantialerasure of the image and a high intensity light at the wavelength ofgreatest bleaching eiciency is preferred for bleaching.

FIG. 3 is a graphical representation of the absorption characteristic ofcalcium fluoride doped with divalent cerium. This material in the stateprior to coloring with an electron beam (as shown in Curve 5) isrelatively transparent to visible light and possesses an absorption bandwhich peaks at about 4000 A. When either light in the band around 4000A. or an electron beam impinges on the calcium fluoride crystal, theabsorption characteristics change to that shown in Curve 6, leaving avisible image on the crystal due to an increase in absorption in awavelength ban from about 4800 A. to about 6400 A. The absorptioncharacteristic, as shown in Curve 6, makes the crystal appear green tothe eye under white light conditions. This absorption characteristic canbe erased by shining intense green light upon the crystal.

In FIG. 4 a cathode ray tube 10 having a screen 11 comprised of aphotochromic material as disclosed herein, is shown. The cathode raytube comprises an evacuated envelope 12 formed with a bulb portion 13and a neck portion 14 extending at an angle to the axis of the bulbportion 13 as shown. Within the bulb portion 13 of the tube 10 isapplied a crystalline film of a suitable photochromic material 11 suchas calcium titanate doped with divalent iron and having thecharacteristics as described above.

The photochromic film or screen 11 is put down upon a flat opticallytransparent portion 15 of the bulb 13. The opposite wall 16 of the bulb13 is also a at portion and optically transparent to permit light topass undistorted therethrough. Within the neck portion 14 of the cathoderay tube 10, is an electron gun structure 17 for forming and focusing acathode ray beam upon the photochromic screen 11. The electron gunstructure 17 may be of any conventional design and is well known in theart. The electron beam formed by the gun structure may be scanned overthe surface of the screen by horizontal deflection coils 18 and verticaldeflection coils 19 to provide the horizonatl and vertical scansion. Thehorizontal and vertical deflection coils 18 and 19 are respectivelyconnected to appropriate circuits as is well known in the art.

The various electrode terminals 21 of the gun are connected, as shown,to a D.C. voltage supply 22 to provide appropriate operating voltages tothe gun structure. The cathode ray tube 10 is connected through a signalreceiver 23 to the voltage supply 22 to provide an operating voltage formaintaining an appropriate cutoff voltage for the electron beam. Thereceiver may be of any type to modulate the cathode ray beam of the tube10.

A source of radiation 31 provides an emission of visible radiationincluding radiation within the electron beaminduced absorption band ofthe screen. The radiation is projected through the transparent bulb wallupon the screen. This source of radiation can be a white light tungsstenbulb. In this structure the photochromic screen 11 is preferablytransparent to light in its unexcited state so that the radiation fromthe radiation source 31 will be transmitted through the screen to aviewer 32 positioned on the same side of the tube as the Screen. Such ascreen can be made from single crystal photochromic material,transparent evaporated layers or transparent hot pressed layers of thephotochromic material or by having the photochromic material imbedded ina glass or plastic having the same index of refraction as thephotochromic material so as to prevent scattering of light from thesurfaces of individual photochromic particles comprising the screen.Generally, the photochromic screen need not be made greater than thepenetration depth of the electron beam. This depth is a function of beamvoltage and density of the photochromic screen. In operation of theembodiment as shown in FIG. 4, a desired signal voltage applied by thereceiver 23 to the electron gun 17 will cause the electron beam tocreate visible traces on the photochromic screen. Signal voltages whichmodulate the electron beam while the beam is scanned by the coils 18 and19 can create a predetermined desired image on the-screen 11 by changingthe absorption charactreistics of selected areas of the screen. Theimages thus formed can then be either selectively or completely erasedby light in the absorption band of an intensity greater than that fromthe radiation source 31. For example, radiation of the desired frequencyfrom a laser 33 may provide erasure. This radiation canv be scanned bymeans known in the art to provide selective erasure. Alternatively, alight source such as a high intensity flood light may be used toaccomplish erasure. The structure of the cathode ray tube as shown inFIG. 4 may be termed the transmissive mode or structure of the device.However, the embodiment of a cathode ray tube, as shown in FIG. 5, ispreferable for most purposes. In this tube 40, a photochromic screen 41is supported by an optically transparent facepalte 42. A reectivecoating 43 may be desposited coextensively with the screen 41, as shown.The screen 41 is comprised of a finely-divided powdered photochromicmaterial which reflects light due to Scattering of the light by thepowder. When the screen is comprised of such a powdered photochromicmaterial, a viewer observes traces or images on the screen by means ofreflected light rather than by means of transmitted light as describedabove. The particle size of the powder should generally be less thanabout 5 microns and preferably be about 1 micron. The screen 41thickness is preferably about l0 microns thick. The screen 41 can befabricated in the same manner as phosphor screens for cathode ray tubes.Such methods are well known in the art and need not be discussed herein.With this tube, the light source should be on the same side of thescreen as the viewer, namely in front of the screen.

The face 42 of the tube 40, which supports the rotochromic screen shouldbe optically transparent to light in the absorption band of the excitedphotochromic material. An image formed on the screen 41 can be erased byshining light upon the screen within the absorption band of thephotochromic material. Preferably, the light used for erasure is of highintensity due to the fact that the eficiency of erasing is less thanthat for writing. The image formed on the screen can either be totallyerased or in the alternative selected portions of the image can beerased by, for example, means of a liber optic light pen 44 whichdirects the erasing light to such selected portions. One reason why acathode ray tube having a powdered photochromic screen is preferable ascompared with the tube type shown in FIG. 4 is that a higher contrastratio and a darker appearing image can be formed on the powdered screen.This is due to the fact that internal reection of the light in thepowder particles gives the light an effective longer absorption path andhence a greater optical density.

Some of the non-absorbed light is normally lost due to transmittance orscattering in a direction away from the viewer. This results in a lossof contrast ratio and brightness of the image screen. This loss can besubstantially reduced by including the reflecting layer 43, such as anevaporated aluminum film behind the photochromic screen as shown in FIG.5.

The photochromic materials having the highest contrast ratios betweenits bleached state and its image induced state are generally preferablyfor use as a cathode ray tube screen. Of the materials disclosed herein,sodalite containing from about to 2000 p.p.m. of iron and preferablyabout 1000 p.p.m iron, and calcium titanate containing from about 100 to2000 p.p.m. of iron and molybdenum are preferred. The latter beingpreferred due to the black image formed by an electron beam therebyresulting in a black and white picture.

Referring to FIG. 6, the cathode ray tube 60 includes a layer 61 of acathodoluminescent phosphor disposed on the photochromic layer 41, asshown. The phosphor layer 61 emits light in response to electron beamimpingement of the phosphor layer 61 of a wavelength within the inducedabsorption or read band of the photochromic layer 41.

In operation of the tube 60, electron gun voltages are adjusted toproduce a high voltage electron beam which penetrates the phosphor layer61 and causes darkening on the photochromic layer 41 and a lower voltageelectron beam which does not penetrate the phosphor layer' 41 butinstead causes emission of the phosphor. The electron beam is made toscan both the photochromic layer 41 and the phosphor layer 61. The lightemitted by the phosphor layer 61 is the light used to read the imagerecorded on the photochromic layer 41. This is the same wavelength lightthat causes erasure of the image.

Alternatively, electron beams for operating such a tube may be developedyby the use of two electron beam guns in the tube structure. Here, onegun would be adjusted at a relatively high voltage so as to develop abeam which would penetrate the phosphor layer and write images on thephotochromic layer while the second gun would be adjusted to a voltagewhich would cause phosphor emission and would not significantlypenetrate into the photochromic layer.

The phosphor in such a tube must be matched to the particularphotochromic layer used. The phosphor layer is preferably relativelythin and generally is in the order of about 1 to 10 microns thick. Inaddition, it is ad vantageous for the phosphor to be of the fast decaytype so as to reduce erasure of the image due to the read light and toprevent picture smearing. Preferably, the phosphor has a decay rateequivalent to or greater than the elemental image scan rate of theelectron beam. Generally, this is in the order of about 1041 seconds orless. It is also preferable to back the phosphor layer with a reflectivecoating such as aluminum so as to efficiently utilize the light emittedby the phosphor. An example of one suitable phosphor-photochromiccombination in a tube of this type is the combination of a galliumphosphide or cerium doped yttrium aluminum garnet phosphor with asodalite photochromic layer.

As previously indicated, a cathode ray tube of this type can be used inconjunction with a detector 62 which in turn can feed information storedon the cathode ray tube back to a computer. The same computer can beused to up-date the information on the cathode ray tube.

What is claimed is:

1. An electron discharge device comprising an evacuated envelope, aphotochromic screen mounted within said envelope, said screen formed ofat least one photochromic material chosen from the group consisting ofalkaline earth lluorides containing a rare earth ion impurity selectedfrom Ce, La, Tb and Gd, and alkaline earth titanates containingtransition metal ion impurities and having an electron beam inducibleand photoninduced charge transfer transition eradicable absorption bandin the visible regions of the spectrum, and electron beam means withinsaid envelope for producing an absorption pattern on said screen.

2. The device recited in claim 1 wherein said photochromic screencomprises calcium fluoride containing an impurity ion selected from thegroup of divalent rare earth ions consisting of cerium, lanthanum,gadolinium and terbium.

3. The electron discharge device recited in claim 1 wherein thephotochromic screen is comprised of a titanate selected from the groupconsisting of calcium titanate and strontium titanate and wherein saidtitanate contans small proportions of at least one transition metal ion.

4. The device described in claim 3 wherein the transition metal ion isat least one member of the group consisting of iron and molybdenum.

5v. The device described in claim 3 wherein the screen is comprised ofpowdered calcium titanate containing small proportions of iron ions.

6. The electron discharge device recited in claim 1 wherein saidphotochromic screen is comprised of nely divided photochromic powder.

7. The device recited in claim 6 wherein said powder particles have anaverage particle size of less than 5 microns and wherein said screenincludes a reective layer behind said screen.

8. The device recited in claim 7 wherein said powder particles have anaverage particle size of about 1 micron and said screen formed from saidpowder has a thickness of about 10 microns.

9. The electron discharge device recited in claim 1 including areflecting layer contiguous with and behind said photochromic screen.

10. In a dark trace cathode rayy tube Comprising (a) an evacuatedenvelope,

(b) a transparent screen support,

(c) a viewing screen on said support and within said envelope, saidscreen comprised of a layer of photochromic material which exhibitsdarkening thereon in response to electron impingement thereon and whichdargening is eradicable by a photon induced electron charged transfertransition induced by light of a given frequency,

(d) means for projecting an electron beam onto said screen to produce adarkening of selected areas thereof, and

(e) means for reading images stored on said screen, said reading meansbeing Iwithin said envelope, the improvement wherein said reading meanscomprises a cathode-luminescent phosphor layer having a decay time ofless than 10"7 seconds and characterized in that it emits light in avisible read band of said photochromic screen, and electron beam meansfor causing said emission of said phosphor layer.

11. In combination, a dark trace cathode ray tube as described in claim10 and an image detector coupled thereto.

References Cited UNITED STATES PATENTS 2,416,574 2/1947 Fonda 313-912,432,908 12/ 1947 Leverenz 313-91 3,253,497 5/1966 Dreyer S13-91X3,452,332 6/1969 Bron et al 178-7.87X 3,453,604 7/ 1969 Geusic et alS13-92X ROBERT SEGAL, Primary Examiner V. LAFRANCHI, Assistant ExaminerU.S. Cl. X.R.

